A thickness measuring device is an instrument for measuring a medium such as sheet medium including a paper or a banknote, and therefore, the thickness measuring device is required to have a high measuring accuracy.
Referring to FIG. 1, an existing measuring device mainly includes a thickness sensor, a location voltage conversion unit, an A/D converter and a control processor. The thickness sensor is configured to detect a thickness of a detected medium, and the control processor is configured to process thickness data of the detected medium and may be connected to an external host device. In operation, the thickness sensor converts the thickness of the detected medium into an output of displacement. In a case where no medium passing the thickness sensor is detected, an output location of the thickness sensor is X0. In a case where a medium with a thickness H passes the thickness sensor, the output location of the thickness sensor is X1. A displacement ΔX=X1−X0 is directly proportional to the thickness H (or the displacement and the thickness have another determined conversion relationship). If a measuring range for the thickness is 0−Hmax, output locations corresponding to the measuring range are X0, Xmax, respectively.
The location voltage conversion unit is configured to convert a location signal outputted from the thickness sensor into an output of voltage signal. In a case where an inputted location is X0, an output voltage U0 corresponding to X0 is referred to as a zero voltage. In a case where the inputted location is Xmax, an output voltage Umax corresponding to Xmax is referred to as a maximal output voltage. It is assumed that a minimal input voltage and a maximal input voltage of the A/D converter are Vmin and Vmax, respectively, a relationship Vmin≦U0<Umax≦Vmax must be met to make sure normal operation of the measuring device. And the measuring device operates in an optimal state in a case that Vmin=U0 and Vmax=Umax.
The above thickness measuring device has the following disadvantages.
I. By Limitation of mechanical machining accuracy and assembly accuracy, output locations X0 of different thickness sensors may fluctuate within a certain range in a case where no medium passes the thickness sensors.
II. In a case that the thickness sensor and the location voltage conversion unit are assembled together, there must be a certain drive mechanism which couples the output location of the thickness sensor to an input terminal of the location voltage conversion unit, since a process of coupling is also affected by the mechanical machining accuracy and the assembly accuracy, the actual location coupled to the input terminal of the location voltage conversion unit is changed, thereby changing the output voltage U0.
III. In a case where multiple thickness measuring devices are installed in one machine, the above factors I and II may cause unconformity between the measuring devices.
IV. The output location X0 and the zero voltage U0 may be changed slowly as an accumulated operating time of the thickness measuring device becomes longer.
An sensitivity of the thickness sensor for the thickness of the detected medium is generally not high, mostly about 1:1, due to high detecting speed and limitation of factors such as dynamic characteristics of a mechanical system. In a case where the detected medium is valuable securities such as a banknote or a check, a thickness of the medium is generally about 100 μm. Then a change value of an output location of the thickness sensor is also about 100 μm. In a case that a measuring error is required to be less than 10%, a total error for the mechanical machining and the assembly is required to be not greater than 10 μm, which is apparently a unreachable requirement for machining and assembling mechanical parts.
Currently, a method for solving zero voltage drift is as follows. In a case that it is predicted that a zero voltage U0 gradually decreases after leaving a factory, the zero voltage is set to be greater than the minimal input voltage of the A/D converter, and the maximal output voltage Umax is set to be equal to the maximal input voltage of the A/D converter in the production. In a case that it is predicted that the zero voltage gradually increases after leaving the factory, the zero voltage is set to be equal to the minimal input voltage of the A/D converter, and the maximal output voltage is set to be less than the maximal input voltage of the A/D converter when leaving the factory. In a case that a changing direction of the zero voltage can not be determined or the zero voltage may increases or decreases, the zero voltage needs to be greater than the minimal input voltage of the A/D converter and the maximal output voltage needs to be less than the maximal input voltage of the A/D converter. In most of actual products, the zero voltage may increases or decreases. After leaving the factory, in a case where the zero voltage gradually decreases, the zero voltage may not be less than the minimal input voltage of the A/D converter since a descending space is reserved previously; in a case where the zero voltage gradually increases and a medium passes the thickness sensor, an amplitude of the output voltage of the location voltage conversion unit may be closer to the maximal input voltage of the A/D converter, and the output voltage may not exceed the maximal input voltage of the A/D converter since an ascending space is reserved previously.
In the above method, although the problem that the output voltage of the location voltage conversion unit exceeds the range of input voltage of the A/D converter is solved, the reserved voltage drift space reduces a changing range of a thickness voltage, thereby reducing sensitivity and distinguishability of the measuring device which negatively affects thickness signal analysis by a controller.
For example, it is predicted that the zero voltage of the thickness measuring device drifts in a maximal range from −0.5V to +0.5V, the minimal input voltage of the A/D converter is 0 and the maximal input voltage of the A/D converter is 3.3V. In this case, when leaving the factory, the zero voltage shall be set as 0.5V, and the sensitivity of the measuring device shall be adjusted, such that the maximal output voltage must be less than 2.8V in a case where a thickness of a detected medium is Hmax. Therefore, an amplitude of a net input voltage signal representing the thickness of medium is only 2.3V, and a thickness sensitivity corresponding the amplitude is 2.3/Hmax. A range of the input voltage of the A/D converter is 0-3.3V, and therefore, an ideal value of the thickness sensitivity may be 3.3/Hmax. It can be seen that, the sensitivity of the thickness measuring device is decreased to 70% of the ideal value due to the zero voltage drift.